This invention relates to antibiotic susceptibility testing and more particularly a test assembly of a series of rectangular wells in which wells are placed various concentrations of various antibiotics or therapeutic control agents to ascertain the interaction between such control agents and various microorganisms, usually pathogenic. With the proliferation of antibiotics and other drugs both in the hospital and in the laboratory as well as educational insitutions there is an increasing demand for information concerning the susceptibility or sensitivity of a particular microorganism to various antibiotics or drugs, as well as information on the assay of particular constituents in blood, or other biological liquids.